Does frontal-plane asymmetry in compensatory postural responses represent preparation for stepping?

The bilateral symmetry of feet-in-place responses to postural perturbations in the anterior-posterior direction has not been well studied. This paper presents evidence that right- and left-leg responses that appear to be approximately symmetrical in the sagittal plane may actually involve an asymmetry in the frontal plane, namely, a lateral weight shift. Comparison with trials where subjects stepped suggests that these lateral weight shifts represent early preparations for stepping responses that are aborted before the foot is actually lifted. Thus, it would seem that compensatory stepping involves a sequence of discrete modifiable stages, rather than a single immutable motor program. Moreover, postural responses that appear to be similar in the sagittal plane may actually be seen to involve quite different postural strategies, i.e. in terms of preparation for stepping, when viewed in the frontal plane.

Influence of expectation and arousal on center-of-pressure responses to transient postural perturbations.

Central modulation of postural responses to external perturbations appears to be a function of subject expectation, which in turn depends on: 1) prior experience within the testing paradigm, and 2) available information specific to the impending perturbation. Conceivably, the effects of expectation could be mediated, in part, through arousal-related mechanisms. This study investigated the influence of expectation of "postural threat" on center-of-pressure responses to transient translational platform perturbations in 23 healthy young males. The influence of prior experience was assessed first by performing a series of repeated trials using moderately large perturbations. The effect of providing specific prior information was then evaluated by administering small "low-threat" and large "high-threat" perturbations, with and without prior precued information about the perturbation magnitude. Also assessed were the associations between the postural responses and changes in arousal, the latter indicated by heart rate and skin conductance measures. The results showed that prior testing experience had a profound influence on postural strategy selection and execution, whereas the influence of precued prior information was rather more subtle. A tendency to lean forward prior to perturbation was influenced both by experience and prior information. Marked decreases in arousal occurred over the course of the earliest trials, and there was also some evidence that prior information affected arousal. Although causal relationships cannot be established, the results did show evidence of association between arousal and postural responses, suggesting that arousal could have affected postural strategy selection and execution.